In computing devices connected in accordance with the IEEE 1394 family of serial bus protocols, a circuit is formed on a shared cable from a power supply line, a return path that grounds the circuit, and data lines going to a transceiver. When the computing device is connected by a cable to another device, called a target device, data can be transferred between the two devices. The cable connects to another power supply and return path on the target device. Mechanically, it is desirable to make the ground connection first, then the power supply connection, and then the data lines connection. However, because of specific design problems, this order of connection is not always possible. Also, defective or dirty connectors and twisted cables can prevent this order of connection. When this desired order of connection is altered, current flow is changed and current runs through the transceiver. In some cases this additional current causes thermal overload that damages or destroys the transceiver.
A conventional approach to thermal overload issues in circuits is to provide a ground fault interrupter (GFI) in the circuitry of a device. A GFI utilizes a sensor that looks for balance between the AC signal going out over one twisted pair and the AC signal on the return path. While the device is operating, if the connecting cable is twisted and the sensor detects loss of current, the GFI shuts down the circuit, protecting components from permanent damage.
However, in the realm of portable computing devices described above, a conventional GFI is of no use, because the connecting cable has another connection (the shield on shielded cable) that goes to a separate ground connected elsewhere in the system. In order for convention GFIs to work in such an implementation, multiple GFIs would need to be placed at various locations and separate AC signals would have to be calculated into a single value. This also would require a resistor be placed somewhere in the shield. However, placing a resistor in the shield would violate a design constraint of the IEEE 1394 family of serial bus protocols, which requires the lowest resistance possible in the circuit existing on the shared cable. Thus there is a heartfelt need for a protection circuit that can protect transceivers from excessive current overload and failure.